KCl Crystals Research Articles

Clusters and their properties in aqueous solutions of KDP, KCl and sugar

Clusters in aqueous solutions of KCl and sugar were observed by a laser scattering ultra-microscope (LSUM), which provided direct evidence of the existence of clusters. The size, numerical density and electrical features of clusters in saturated solutions of KCl and KDP were characterized by varied light scattering methods. The numerical density of clusters decreased with the size increment and most of the clusters were much smaller than 100 nm in diameter. The average size was about 1.3 times as large as their unit cells of KDP and KCl crystals in the saturated solutions after filtration with a 450 nm pore filter. These large clusters of KDP and KCl were uncharged molecular particles.

Correlation between deformation bleaching and mechanoluminescence in coloured alkali halide crystals

The present paper reports the correlation between deformation bleaching of coloration and mechanoluminescence (ML) in coloured alkali halide crystals. When the F-centre electrons captured by moving dislocations are picked up by holes, deep traps and other compatible traps, then deformation bleaching occurs. At the same time, radiative recombination of dislocation captured electrons with the holes gives rise to the mechanoluminescence. Expressions are derived for the strain dependence of the density of colour centres in deformed crystals and also for the number of colour centres bleached. So far as strain, temperature, density of colour centres, E a and volume dependence are concerned, there exists a correlation between the deformation bleaching and ML in coloured alkali halide crystals. From the strain dependence of the density of colour centres in deformed crystals, the value of coefficient of deformation bleaching D is determined and it is found to be 1.93 and 2.00 for KCl and KBr crystals, respectively. The value of (D+χ) is determined from the strain dependence of the ML intensity and it is found to be 2.6 and 3.7 for KCl and KBr crystals, respectively. This gives the value of coefficient of deformation generated compatible traps χ to be 0.67 and 1.7 for KCl and KBr crystals, respectively.

The Pb2+(Li) and Pb2+(Na) Centres in KCl Crystals

Emission, absorption and magnetic circular dichroism spectra have been studied on Li+- and Pb2+-codoped KCl crystals. Two emission bands (one is the AT-like emission and the other the AX-like emission) have been observed at 350 and 453 nm at room temperature, respectively. These emissions have been observed by excitation in the absorption bands with peaks at 270 and 281 nm. These emission bands are attributed to the Pb2+(Li) centre, where a Pb2+ ion has a Li+ ion at one of the six nearest neighbouring cation sites. Measurements have been also made on Na+- and Pb2+-codoped KCl crystals. Unlike the Pb2+(Li) centre, only the AT-like emission band with a peak at 347 nm due to the Pb2+(Na) centre has been observed. It is suggested that such a difference is due to a difference in the position of the impurity alkali ion substituted for the K+ ion.

Alkali Metal NMR Chemical Shielding as a Probe of Local Structure: An Experimental and Theoretical Study of Rb+ in Halide Lattices

When Rb+ is incorporated into different cubic halide lattices at very low concentrations, the 87Rb NMR chemical shift becomes more shielded by as much as 282 ppm when the cation is changed from Na+ to Cs+. Similarly, in mixed crystals of KCl and RbCl, the average resonance frequency shifts with the degree of incorporation. The 87Rb chemical shift data for each halide show a near-linear correlation with the average alkali metal−halide interionic distance in the different crystals studied, in good agreement with ab initio calculations which show that indeed the chemical shielding has a strong dependence on the nearest neighbor shell of halide anions and their distance from the Rb+. When the Rb concentration is increased as in KCl/RbCl solid solutions, the 87Rb NMR spectra are influenced by a large distribution of K/Rb configurations in the next-nearest-neighbor shell of 12 cations. While the total shift is still largely dependent on the Rb−Cl distance, shells containing both K and Rb atoms give rise to seco...

The dispersion of AgCl in the KCl crystal

The dispersion of AgCl in the KCl crystal

Effect of shock compression of solid insulators on the injection of valence electrons in strong magnetic fields

A correlation was shown to exist between the velocity of impulsing discharge from the anode, shock-wave parameters, electric-field strength, elastic properties, and the probability of ionization in the valence band of insulating solids. Using NaCl and KCl crystals as examples, a quantitative estimation of these dependences was performed for the interval of discharge speeds of 5×103–106 m/s.

KCl crystallization within the space between carbon nanotube walls

The arc discharge method has been used hitherto to encapsulate metal-containing structures within carbon nanotubes [1, 2], the arrangement of the nanotube walls playing an important role in the process. We show that KCl can be generated in the space between nanotube walls by treating potassium-intercalated uncapped tubes with CCl 4. A relationship between the KCl crystal domain and the hexagonal carbon network is thought to give rise to preferential formation of a 100 plane perpendicular to the tube axes.

Mass-Spectrometric Study of the Temperature Variation in the Dimer-to-Monomer Ratio in the Free-Surface Vaporization Fluxes from Alkali Halide Single Crystals

A mass-spectrometric method is used to investigate the kinetics of the vaporization of LiF (970–1070 K), NaCl (722–889 K), KCl (780–900 K), KBr (724–918 K), and CsI (656–838 K) single crystals. A dimer-to-monomer ratio, JD/JM, in the fluxes vaporizing from free surfaces of these crystals are calculated, using the currents of M+, MX+, and M2X+ ions formed via electron impact ionization of the molecules MX and M2X2 (M is the alkali atom; X is the halogen). It is found that the dimer-to-monomer ratio increases with temperature at a continually increasing rate in the LiF and NaCl cases. In the KCl, KBr, and CsI cases, this rate passes through a maximum. Such a specific temperature variation in JD/JM is discussed through the analysis of proposed mechanisms of dimer formation in light of the terrace–ledge–kink and surface charge models.

Heteroepitaxial strain in alkali halide thin films: KCl on NaCl

We have performed Monte Carlo simulations of the properties of a NaCl (001) surface covered by full or partial layers of KCl, for coverages up to 5 monolayers (ML). A wide variety of structures of the film is found. For integer ML coverages we find the continuous, so-called floating mode rumple structure, as was previously found in the KBr/NaCl system. However, for a coverage of $\ensuremath{\sim}2.1\mathrm{ML},$ we find a discrete structure of periodicity 3:4 of small regularly spaced KCl pyramids. It has the same scattering characteristics as the structure observed by Henzler et al. [Phys. Rev. B 52, 17 060 (1995)], but it is two-dimensional modulated, rather than the rowlike stacking fault structure proposed by Henzler et al. Also, at a coverage of $\ensuremath{\sim}0.8\mathrm{ML}$ there is a stable 3:4 structure. Other structures are found at intermediate coverages, corresponding to regular arrays of dislocation lines with periodicity 9:10. A further growth from such structures would give rise to growth of a $\ensuremath{\sim}5\ifmmode^\circ\else\textdegree\fi{}$ miscut KCl crystal relative to the NaCl lattice. The reciprocal space patterns corresponding to the continuous and the discrete film deformations are also calculated.

Dislocation spectroscopy of crystals

A new method of studying the energy characteristics of dislocations is proposed, which is based on the investigation of the interaction of moving dislocations with purposefully introduced electronic and hole centers. A study has been made of KCl, NaCl, KBr, LiF, and KI alkali halide crystals containing electronic F and hole VK and Me++ (Cu++, Ag++, Tl++, In++) centers. Investigation of the temperature dependence of the dislocation interaction with the F centers permitted determination of the position of the dislocation-induced electronic band (DEB) in the band diagram of the crystal. In KCl, the DEB is separated by ≈2.2 eV from the conduction-band minimum. It is shown that dislocations transport holes from the centers lying below the dislocation-induced hole band (DHB) (X+, In++, Tl++, VK) to those above the DHB (the Cu+ and Ag+ centers). Such a process is temperature independent. The DHB position in the crystal band diagram has been determined; in KCl it is separated by ≈1.6 eV from the valence-band top. The effective radii of the dislocation interaction with the electronic F and hole X+, VK, and Tl++ centers have been found.

Electronic processes of f-centers in KCl crystals by L1-light excitation

F-centers in an additively colored KCl crystals were excited by light of the L1-band (3371 Å). The emission spectrum of the main bands were measured at several temperatures ranging from 47 to 171 K. At least three bands were clearly identified, the F*-band, the K-band, and the M-band. A mechanism for the electron decay which leads to an interpretation of the data is proposed. In addition, the experiment suggests that under L1-light excitation, the luminiscence lifetime may contain at least three lifetimes: the lifetime of F* (∼600 ns), the lifetime for the K-band (335 ns) (Da Silva, Phy. Stat. Sol. (b) 169 (1992) K119–122) and the one corresponding to the M-center (30 ns).

Mobility and currents of F-centers in electrolytically colored KCl crystals

At room temperature, alkalihalides are dielectric solids, while at high temperatures, they present electrolytic conductivity. As the current flows through the crystals, a simultaneous coloring takes place. An electronic conductivity is also observed during this process. Different F-center diffusion mechanisms are observed, depending on the temperature range. In this work, KCl crystals have been electrolitically colored. When current and optical transmissions are measured at the same time, an almost simultaneous coloring is detected within the first zone of current. At temperatures under 778 K, the mobility suffers a substantial drop, in a way that resembles what is suggested for Kl crystals. This suggests the possibility of a different diffusion mechanism at low temperature.

Electronic States and Optical Absorption Spectra ofHCenters in Potassium Halide Crystals

Irradiation of undoped KI single crystals with a KrCl excimer laser ( h ν=5.58 eV) near liquid helium temperature results in the production of α, β, F , F ′ and H centers. The H centers, i.e., I 4 3- molecular ions in KI, give rise to two prominent optical absorption bands peaking at 2.23 and 2.77 eV. The theoretical results for the (halogen) 4 3- molecular ion, which are obtained by using the semiempirical self-consistent field method, show that the 2.23- and 2.77-eV bands are ascribed to the transitions of the ground 2 Σ u 1/2 + state to the excited 2 Π g 1/2 and 2 Σ g 1/2 + states, respectively. The same assignments are made for two distinct absorption bands arising from H centers in KCl and KBr crystals. The relative intensities of the two distinct bands for KCl, KBr and KI crystals depend on the magnitude of the spin-orbit interaction in halogen atoms. The peak energies and intensities of absorption bands caused by H centers in the potassium halide crystals are discussed in comparison between experi...

Conductivity of ionic crystals when they are irradiated by picosecond electron beams

This paper discusses the pulsed electron conductivity σ of KCl, KBr, and NaCl crystals when they are excited by an electron beam (0.2 MeV, 50 ps) with current densities in the interval j=(30–104) A/cm2. It is shown that the lifetime of the electrons in the conduction band is τ≪100 ps. To explain the experimental σ(j) dependences, a model is proposed that includes electron capture by structural defects and stable radiation defects at low excitation densities and electron capture predominantly by unstable radiation defects generated by the excitation pulse at high excitation densities.

The effect of temperature on the pulsed conductivity of a KCl crystal excited by picosecond electron pulses

This paper discusses the temperature dependence of the pulsed conductivity of a KCl crystal in the interval 12–300K when it is excited by an electron beam (0.2 MeV, 50 ps, 300A/cm2) with a time resolution of 150 ps. It is shown that the electron lifetime is τ<100 ps in the entire interval under consideration, while the conductivity increases with temperature. The experimental results make it possible to obtain the temperature dependences of the effective electron-hole recombination cross section S∼T3.5 and the separation probability of genetic pairs.

Formation and stabilization of F centers following direct generation of self-trapped excitons in KCl crystals

The spectrum of luminescent F centers generated in high-purity KCl crystals by 7–10.2-eV photons has been measured at 230 K. The pulsed annealing of these centers (250–550 K), as well as the dependence of the efficiency of stable F-center generation on irradiation temperature (80–500 K) has been studied. The efficiencies of F− and Cl3−-center generation are maximum under direct optical creation of self-trapped excitons in the region of the Urbach intrinsicabsorption tail. Besides the exciton decay with formation of F centers and mobile H centers, a high-temperature exciton decay channel which involves creation of cation defects stabilizing the H centers has been revealed.

Dielectric relaxation of interacting hydroxyl ions in KCl crystals

We report low-temperature measurements of the dielectric susceptibility and dielectric absorption of OH − and OD − defects in potassium chloride crystals. With rising concentration mutual interaction between the defects leads to a cross-over from coherent to incoherent tunnelling, which in turn changes the energy spectrum and the dynamics of the defects. In addition, we have studied the dielectric absorption arising from the thermally activated reorientation of well-defined defect pairs.

Sound velocity and internal friction of Li-doped KCl

Low-frequency acoustic experiments are reported on Li-doped KCl crystals and pure KCl at temperatures between 70 mK and 30 K. The temperature dependence of the transverse sound velocity is caused by the eight-level Li-tunneling systems and their particular symmetry. Both resonant and relaxational contributions to the acoustic susceptibility can be identified. A consistent interpretation is possible for the two isotopes 6Li and 7Li.

Recrystallization of pure and strontium-doped KCl crystals

The kinetics of microstructure transformations are studied during annealing of deformed single crystals of KCl and KCl:0.05wt %Sr2+ at temperatures of (0.35–0.55)T m (where T m is the melting temperature) and during storage at room temperature. The effect of deformation rates ranging from 0.01 mm/min to 0.1 mm/min at a deformation temperature T d=0.5T m on the crystal structure and on the recrystallization kinetics is noted. It is found experimentally that the incubation period for static recrystallization in single-crystal KCl:0.05wt%Sr2+ is shortened and recrystallization takes place at room temperature after deformation in this temperature range. Here, during the new recrystallization grains have a twinned orientation with respect to the initial single crystal during the first stage and to the subgrains of the deformed crystal. As the annealing temperature is raised, the stage in which twins grow in KCl:0.05wt%Sr2+ crystals is shortened and it is displaced by recrystallization through migration of high-angle grain boundaries of the common type. Deformation conditions which ensure prolonged (at least three months) stability of the post-deformation hardening of single crystals are found experimentally for Sr2+-doped deformed single crystals.

Optically stimulated luminescence in KCl:Cu x-irradiated at room temperature

Optically stimulated luminescence (OSL) has been observed in single crystals of KCl:Cu x-irradiated at room temperature. It is shown that electrons are liberated from anion sites during the OSL process. The OSL predominantly involves emission due to radiative transition (d↔s) of monovalent copper ions present in the lattice. The OSL emission shows a strong temperature dependence indicating a thermally assisted process. Electron-hole recombination followed by energy transfer to the Cu activator is suggested as a possible OSL mechanism in KCl:Cu. Preliminary results of OSL in KBr:Cu are also presented.